The Global Atmosphere Watch Programme (GAW) of the World Meteorological Organization (WMO) has a unique long-term international framework that provides the technical basis for integrated observations, analysis and assessment of atmospheric chemical composition. The missions of the GAW, as described in the “WMO Global Atmosphere Watch (GAW) Strategic Plan: 2016 - 2021” (GAW Report No. 172), are to implement “maintaining and applying global, long-term observations of the chemical composition and selected physical characteristics of the atmosphere; emphasizing quality assurance and quality control; and delivering integrated products and services of relevance to users."
The WMO/GAW quality assurance system includes five types of central facilities dedicated to six groups of measurement variables, which are operated by WMO Members. Its key role is played in the quality assurance system by World Calibration Centres (WCCs), which assists WMO Members in assuring the quality of observations and in particular in establishing observations traceable to the WMO/GAW Reference Scale.
KMA (Korea Meteorological Administration) has operated the World Calibration Centre for Sulphur hexafluoride (WCC-SF6), one of the central facilities in the World Meteorological Organization / Global Atmosphere Watch (WMO/GAW) programme, since October 2012.
The duties of WCC-SF6 as follows:
a) assist WMO members operating WMO/GAW stations to link their sulphur hexafluoride (SF6) observations to the WMO/GAW
reference Scale through comparisons with standards calibrated against the primary/secondary standards maintained
by the Central Calibration Laboratory for SF6;
b) assist the WMO/GAW Scientific Advisory Group (SAG) on Greenhouse Gases in the development of the quality control
procedures required to support the quality assurance of SF6 measurements and ensure the traceability of these
measurements to the corresponding primary standard;
c) maintain laboratory and transfer SF6 gas standards that are traceable to their respective primary standard(s);
d) perform regular calibrations and inter-comparison campaigns involving the WMO/GAW stations/laboratories;
e) assist in provision of training and long-term technical help for WMO/GAW stations; and
f) make public its involvement in the WMO/GAW Programme (e.g. on its websites, in its newsletters, etc.).
-Working and Travelling standards
WCC-SF6 established several systems for sampling compressed dry natural air and analyzing SF6 to prepare laboratory and transfer gas standards traceable to the WMO/GAW SF6 Reference Scale.
-Cleaning cylinders: 10 L aluminum cylinders are cleaned up with evacuation system, a turbo molecular pump and a heating apparatus more than 12 hours that is able to maintain the residual pressure inside the cylinder less than 0.1 Pa. (Fig. 1(a)).
-Collecting air: Dry natural air sample is collected by an oil free air compressor through a dekabon tube from the top of 40 m height sampling tower. To remove moisture in the air sample, a chemical trap filled with granulated magnesium perchlorate is used. (Fig. 1(b_,(c),(d))
-Analysing SF6: After preparation of compressed air in the cylinders, SF6 mole fraction of the samples are assigned by a gas chromatograph with electron capture detector (GC-μ ECD, Agilent 7890) and five reference gases of the WMO SF6 mole fraction scale maintained by the CCL (in NOAA). (Fig. 2, 3)
(a) Cylinder evacuation system (b) Sampling tower
(40 m height)
(c) Air compressor room (d) Air sampling system
Figure 1. Systems for preparation of compressed dry air in the cylinder
(a) Pre-concentrator (b) Analytical system (c) NOAA X2014 scale
Figure 2. Analytical system and WMO reference scale (NOAA X2014) to prepare for labotory and transfer standards gases, which are traceable to the WMO reference scale
-Method description: Preconcentrator + Back flush
GC-μ ECD (Agilent 7890) is used to assign mole fractions. Configuration of analytical system is as follows:
Gas cylinder → regulator → MFC (Mass Flow Controller) → Pre-concentration → sample injection valve (back flush) → column → detector
Detector μ ECD Detector temperature 375 °C Column Alumina 6ft (pre), Alumina 18ft (main) Sample Flow 100 mL/min Carrier gas, pressure P5(Nitrogen/Methane), 60 psi Oven temperature 60 °C
Figure 3. Calibration curve (left) and chromatogram (right) of the five NOAA SF6 standard gases
- Valve Position (Preconcentrator + Injection valve)
Figure 4. Valve positions during the analysis of SF6 at WCC-SF6 lab
- - Two-point calibration is implemented against WMO scale (NOAA-X2014) with the sequence of CRM1-sample-CRM2-CRM1 to monitor the instrument drift.